Method of producing a compound roll

ABSTRACT

The compound roll having a shell portion made of a hard, high-alloy cast steel or iron having excellent wear resistance and resistance to surface roughening and a core portion made of a tough cast iron or cast steel, the cast iron of the shell portion having a composition consisting essentially, by weight ratio, of 1.0-3.0% of C, 2.0% or less of Si, 2.0% or less of Mn, 2.0-15.0% of Cr, 10.0% or less of Mo, 2.0-8.0% of V, the balance being substantially Fe and inevitable impurities, an average diameter of crystal grains constituting a matrix of the metal structure of the shell portion being 100 μm or less in a range from a surface to a depth of 50 mm when determined by an image analysis method on the crystal grains having diameters exceeding 30 μm, and the crystal grains satisfying the formula: m 2  ≦1.2 m 1 , wherein m 1  is an average diameter of the crystal grains at the surface of the shell portion, and m 2  is an average diameter of the crystal grains at the depth of 50 mm, is produced by a centrifugal casting method which comprises supplying a melt for the shell portion at a temperature T satisfying the formula: Tc≦T≦Tc+90° C., wherein Tc is a primary crystal-forming temperature of the melt for the shell portion, to a hollow cylindrical mold rotatable around its longitudinal axis, at such a speed that an average shell portion-forming speed in the mold is 2-40 mm/min.

BACKGROUND OF THE INVENTION

The present invention relates to a compound roll comprising a shellportion having excellent wear resistance and resistance to surfaceroughening and a tough core portion, and a method of producing such acompound roll by a centrifugal casting method, particularly to acompound roll having a shell portion having a fine and uniform metalstructure and a method of producing such a compound roll.

In a roll used for hot rolling or cold rolling of steel materials, it isrequired that a shell portion, which is brought into direct contact withmaterials to be rolled, has a uniform cast structure and excellent wearresistance, resistance to surface roughening and crack resistance. Tomeet these requirements, it is effective to form the shell portion by acentrifugal casting method, and the production of such compound rollshaving shell portions and core portions is widely conducted. In thecentrifugal casting method, a melt for a shell portion is usuallyintroduced into a hollow cylindrical mold rotatable around itslongitudinal axis at a high speed, and solidified in the mold.

In this case, since the melt is rapidly cooled by contact with an innersurface of the mold usually made of steel, an outer surface of the shellportion of the compound roll has a fine metal structure. Therefore, theshell portion of the compound roll shows excellent wear resistance,resistance to surface toughening and crack resistance. However, as adistance between the inner surface of the mold and the melt to besolidified becomes larger, the cooling speed of the melt for the shellportion decreases, and the temperature gradient of the melt becomessmaller. As a result, the metal structure of the shell portion becomescoarser, so that various properties required for the shell portion suchas a wear resistance, etc. are deteriorated. Accordingly, after a longperiod of time during which machining of the shell portion of thecompound roll is repeated, an inner portion of the shell portion isexposed outside, which fails to keep the above excellent properties. Tosolve this problem, it is considered to be effective that the coolingspeed of the melt for the shell portion is increased, and it isnecessary to make the cooling speed of the melt as even as possible inany portion along the radial direction of the shell portion.

To increase the cooling speed of the melt for the shell portion,proposals were made to cool the mold by water, and to spray the meltonto an inner surface of the mold (Japanese Patent Laid-Open No.1-254363). Also, to avoid undesirable segregation and other defectsgenerated in the shell portion and to improve the uniformity of theshell portion, it was proposed to move the point of pouring the meltinto the mold in the centrifugal casting method (Japanese PatentPublication No. 50-33021). Further, research has been conducted on thematerials for the shell portion. At present, the shell portion producedby the centrifugal casting method is mainly made of a high-alloy castiron, a high-chromium cast iron, a high-chromium cast steel, etc. Also,recently, a high-speed steel material was proposed to form the shellportion of the compound roll (Japanese Patent Laid-Open No. 60-124407).

Among them, in the case of a roll used for hot rolling or cold rolling,the non-uniformity of the cast metal structure of the shell portion dueto the existence of coarse precipitated grains and segregation leads toa poorer wear resistance, resulting in the increase of roll consumptionper a unit weight of a material to be rolled and poorer quality of therolled material.

Since high quality is increasingly required for rolled steel sheetsrecently, high requirements are imposed on the roll. Therefore, theshell portion of the compound roll is required to have an increasinglyfiner metal structure with higher uniformity.

In the case of forming the shell portion with the high-speed steel, asurface portion of the shell portion has a fine metal structure by therapid cooling action of the mold. However, since the rapid coolingaction of the mold decreases inside the shell portion, the metalstructure becomes coarser. As a result, in the deep area of the shellportion, which is to be exposed by several times of machining, it showspoor resistance to wear and surface roughening.

In the case of forming the shell portion by a centrifugal castingmethod, there is also a problem that the shell portion inevitablycontains cast defects and non-uniformity of the metal structure. Sincethe cooling speed (temperature gradient) of the shell portion is smallerin the inside than in the surface portion, it is difficult for gas,dissolved elements, impurities, etc. in the melt for the shell portionto escape toward an inside mold cavity into which the melt is poured.Accordingly, these components are trapped in the process of solidifyingthe melt, resulting in the segregation of carbides, coarse metalstructure, gas defects, etc.

OBJECT AND SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a compoundroll having a shell portion having a fine metal structure with excellentuniformity.

Another object of the present invention is to provide a method ofproducing such a compound roll.

As a result of intense research on the centrifugal casting method inview of the above objects, the inventors have found that by controllingthe supply temperature of the melt and the shell portion-forming speed,a large cooling speed and so a large temperature gradient can beachieved in the solidification interface between the melt and the shellportion, thereby preventing the excess growth of the metal structure inthe shell portion, which in turn leads to the production of the shellportion free from cast defects.

Thus, the compound roll produced by centrifugal casting according to thepresent invention comprises a shell portion made of a hard, high-alloycast steel or iron having excellent wear resistance and resistance tosurface roughening and a core portion made of a tough cast iron orsteel, the high-alloy cast steel or iron of the shell portion having acomposition consisting essentially, by weight ratio, of 1.0-3.0% of C,2.0% or less of Si, 2.0% or less of Mn, 2.0-15.0% of Cr, 10.0% or lessof Mo, 2.0-8.0% of V, the balance being substantially Fe and inevitableimpurities, an average diameter of crystal grains constituting a matrixof the shell portion being 100 μm or less in a range from a surface to adepth of 50 mm when determined by an image analysis method on thecrystal grains having diameters exceeding 30 μm, and the crystal grainssatisfying the formula: m₂ ≦1.2 m₁, wherein m₁ is an average diameter ofthe crystal grains at the surface of the shell portion, and m₂ is anaverage diameter of the crystal grains at the depth of 50 mm.

The method of producing the above compound roll according to the presentinvention comprises centrifugally casting said shell portion bysupplying a melt for the shell portion at a temperature T satisfying theformula: Tc≦T≦Tc+90° C., wherein Tc is a primary crystal-formingtemperature in the shell portion, to a hollow cylindrical mold rotatablearound its longitudinal axis, at such a speed that an average shellportion-forming speed in the mold is 2-40 mm/min.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the result of a sulfur print test inExample 1;

FIG. 2(a) is a photomicrograph showing the metal structure of the shellportion at a surface in Example 2;

FIG. 2(b) is a photomicrograph showing the metal structure of the shellportion at a depth of 50 mm from the surface in Example 2;

FIG. 3(a) is a photomicrograph showing the metal structure of the shellportion at a surface in Comparative Example 2; and

FIG. 3(b) is a photomicrograph showing the metal structure of the shellportion at a depth of 50 mm from the surface in Comparative Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] Composition of high-alloy cast steel or iron for shell portion

The high-alloy cast steel or iron which may be used for the shellportion of the compound roll according to the present invention has thefollowing composition:

(1) C: 1.0-3.0 weight %

C is an element necessary for forming carbides to increase the wearresistance of the shell portion, but as the amount of C increases, thecrack resistance of the shell portion decreases. Accordingly, the amountof C should be within the range of 1.0-3.0 weight %. If the amount of Cis less than 1.0 weight %, the resulting shell portion would show a poorwear resistance because of small amounts of carbides precipitated. Onthe other hand, if the amount of C is more than 3.0 weight %, theresulting shell portion would show poor crack resistance. The preferredamount of C is 1.3-2.0 weight %.

(2) Si: 2.0 weight % or less

Si is an element necessary as a deoxidizer. It is also effectivelydissolved in M₆ C carbides in place of expensive elements such as W, Mo,etc. thereby reducing the cost of the shell portion. However, if theamount of Si exceeds 2.0 weight %, the resulting shell portion wouldhave cast defects. The preferred amount of Si is 0.3-1.5 weight %.

(3) Mn: 2.0 weight % or less

Mn functions as a deoxidizer and as an element for forming a compoundwith S as MnS so that the adverse effect of S is eliminated. However, ifthe amount of Mn exceeds 2.0 weight %, a residual austenite phase islikely to be formed in the resulting shell portion, thereby failing tostably exhibit sufficient hardness. On the other hand, if the amount ofMn is too small, sufficient deoxidizing function cannot be exhibited.The preferred amount of Mn is 0.3-1.5 weight %.

(4) Cr: 2.0-15.0 weight %

If the amount of Cr is less than 2.0 weight %, sufficient hardenabilitycannot be obtained. On the other hand, if the amount of Cr exceeds 15.0weight %, excess amounts of chromium carbides (M₂₃ C₆) are formed. SinceM₂₃ C₆ is softer than MC and M₂ C, the chromium carbides would reducethe wear resistance of the shell portion. The preferred amount of Cr is3.0-10.0 weight %.

(5) Mo: 10.0 weight % or less

Mo serves to increase the hardenability and high-temperature strength.However, if the amount of Mo exceeds 10.0 weight %, M₆ C and M₂ Ccarbides would increase in the metal structure in balance with C, V andMo, resulting in a poor toughness and resistance to surface roughening.The preferred amount of Mo is 2.0-8.0 weight %.

(6) V: 2.0-8.0 weight %

V is an element necessary for forming MC carbides to increase the wearresistance of the shell portion. If the amount of V is less than 2.0weight %, sufficient effect cannot be obtained. On the other hand, ifthe amount of V exceeds 8.0 weight %, the melt is severely oxidized andso the viscosity of the melt is increased. As a result, a good castshell portion cannot be produced. The preferred amount of V is 2.0-6.0weight %.

(7) Optional elements

In addition to the above elements, the high-alloy cast steel or iron forthe shell portion may optionally contain W, Ni, Co, and/or N.

(a) W: 20.0 weight % or less

W is an element necessary for maintaining the high-temperature strength.However, if the amount of W exceeds 20.0 weight %, M₆ C carbides wouldincrease in the metal structure, resulting in a poor toughness andresistance to surface roughening. The more preferred amount of W is2.0-15.0 weight %.

(b) Ni: 3.0 weight % or less

Ni is effective to increase the hardenability. Accordingly, Ni may beadded in an amount of 3.0 weight % or less. However, if Ni is addedexcessively, there appears a residual austenite phase in the metalstructure of the shell portion, resulting in cracking, and a poorresistance to surface roughening. The more preferred amount of Ni is0.1-1.5 weight %.

(c) Co: 10.0 weight % or less

Co is dissolved in the matrix and retards the precipitation of carbides,thereby preventing the softening of the matrix. Namely, Co is an elementeffective for increasing resistance to temper softening and foreffecting secondary hardening. However, even if the amount of Co exceeds10.0 weight %, the cost of the shell portion would be increased withoutfurther improvement. The more preferred amount of Co is 1.0-7.0 weight%.

(d) N: 0.03-0.2 weight %

In the high-alloy cast steel or iron used in the present invention, theamount of N is preferably 0.03-0.2 weight %. With this range of N, thetempering hardness is increased. However, if N is added excessively, theshell portion would be brittle. The more preferred amount of N is0.03-0.1 weight %.

(8) Impurities

The inevitable impurities of the high-alloy cast steel or iron used inthe present invention are P and S. To prevent the brittleness of theshell portion, the amount of P should be less than 0.1 weight %, and theamount of S should be less than 0.06 weight %.

[2] Centrifugal casting method

The centrifugal casting method according to the present invention isconducted under the conditions of the controlled supply temperature ofthe melt for the shell portion and the controlled shell portion-formingspeed.

(1) Tc≦T≦Tc+90° C.

In the centrifugal casting method, the temperature of the melt for theshell portion measured in a tundish is usually regarded as its castingtemperature. However, the actual temperature of the melt in the hollowcylindrical mold is somewhat lower than the temperature of the meltmeasured in the tundish. To achieve an accurate control of the castingtemperature, the temperature of the melt in the hollow cylindrical moldshould be determined. Although it is generally difficult to measure thetemperature of the melt in the mold, the inventors have found that thereis a correlation between the temperature of the melt just dischargedfrom the outlet of the tundish and the temperature of the melt in themold. This correlation can be determined experimentally depending on thesize and shape of the hollow cylindrical mold, pouring speed of the meltand operation conditions, etc. Accordingly, the temperature of the meltjust flowing from the outlet of the tundish can be controlled to achievethe most preferred casting temperature.

Defining the temperature of the melt just flowing from the outlet of thetundish (entering into the hollow cylindrical mold) as a supplytemperature "T", the supply temperature should meet the requirement:Tc≦T≦Tc+90° C., wherein Tc is a primary crystal -forming temperature inthe shell portion.

The above supply temperature is lower than the casting temperature inthe conventional centrifugal casting method. Accordingly, the austeniticprimary crystals start to be formed and the melt starts to be solidifiedas soon as the melt is introduced into the hollow cylindrical mold.

If the supply temperature T is higher than Tc+90° C., it takes too muchtime until the melt is solidified in the hollow cylindrical mold,failing to provide a large cooling speed. This leads to the excessgrowth of the metal structure of the shell portion, resulting in acoarse metal structure (coarse primary crystals). On the other hand, ifthe supply temperature T is lower than Tc, the melt starts to besolidified in the discharge nozzle of the tundish, failing to theformation of a good shell portion.

Incidentally, the supply temperature T can be determined by subtractingan experimentally obtained parameter (for instance, 10-60° C.) from thetemperature of the melt just exiting from the discharge nozzle of thetundish.

(2) Average shell portion-forming speed

The average shell portion-forming speed is defined herein as a valueobtained by dividing the total thickness of the shell portion formed bythe time consumed. In general, the shell portion-forming speed in ausual centrifugal casting method is set at 50-200 mm/min. in order tomake sure that the melt introduced can be uniformly laid on an entireinner surface of the mold. However, such a high shell portion-formingspeed leads to a small cooling speed of the melt in the mold, whichmeans that a uniform, fine metal structure cannot be obtained in theshell portion.

In the present invention, the average shell portion-forming speed is setat as small as 2-40 mm/min. to make sure that the melt is supplied ontothe surface of the shell portion being formed in the mold atsubstantially the same speed as the advancing speed of thesolidification interface. By controlling the average shellportion-forming speed at this level, a thin melt pool can always be keptinside the shell portion being formed in the mold, and thesolidification interface can advance radially inward without disturbanceand non-uniformity.

Since the thin melt pool has a small heat capacity, a large coolingspeed of the melt pool can be achieved by heat conduction and heatdissipation through the solidified shell portion and the mold. Also,since the melt pool is cooled in the mold from near a primarycrystal-forming temperature to a solid-liquid solidificationtemperature, a large temperature gradient can be obtained. Such largecooling speed and temperature gradient can be achieved inside the thinmelt pool by causing the solidification interface to advance in parallelwith the axis of the hollow cylindrical mold while keeping uniformity.This contributes to the formation of a uniform and fine metal structurewithout cast defects. Such effects would not be obtained if the averageshell portion-forming speed exceeds 40 mm/min., resulting in a coarsemetal structure. On the other hand, if the average shell portion-formingspeed is lower than 2 mm/min., the supply of the melt is so insufficientthat the supply of the melt pool cannot keep up with the advance of thesolidification interface, failing to provide a good shell portion.

Incidentally, in the initial stage of supplying the melt for the shellportion into the mold, the supply speed of the melt may be as high as50-200 mm/min. because the melt in contact with the inner surface of themold is rapidly cooled. This initial stage may be conducted generally upto about 40%, preferably up to about 35% of the total thickness of theshell portion. Thereafter, the shell portion-forming speed should belowered, so that the average shell portion-forming speed becomes 2-40mm/min.

With respect to the core portion, it should be noted that its materialsand production conditions are not restrictive, and that any cast ironand cast steel can be used under known casting conditions as long ashigh mechanical strength such as bending strength, toughness, etc. canbe achieved.

[3] Metal structure of shell portion

The shell portion produced by the above centrifugal casting method has ametal structure in which fine crystal grains are uniformly distributed.The term "crystal grains" used herein means particles or phasesprimarily precipitated in the solidification of the melt of the shellportion, which mainly consist of austenite. The crystal grains aresometimes called "primary crystals."

In the first aspect of the present invention, the fine crystal grainshave an average diameter of 100 μm or less from a surface to a depth of50 mm in the shell portion, when only fine crystal grains havingdiameters exceeding 30 μm are counted in an image analysis method.

Since the crystal grains are in various shapes in a photomicrograph,their diameters cannot be determined without converting the crystalgrains to true circles. Accordingly, they are first converted to truecircles having the same areas as those of the crystal grains by an imageanalysis method, and the diameters of the true circles obtained from thecrystal grains are averaged. In this case, only the true circles havingdiameters exceeding 30 μm are counted, because calculation would beextremely difficult if those having diameters lower than 30 μm areincluded in the calculation of the average diameter.

If the average diameter of the crystal grains calculated by the abovemethod is larger than 100 μm, the metal structure of the shell portionis too rough, failing to produce high-quality rolled steel sheets.

In the second aspect of the present invention, the average diameter ofthe crystal grains satisfies the formula: m₂ ≦1.2 m₁, wherein m₁ is theaverage diameter of the crystal grains at the surface of the shellportion, and m₂ is the average diameter of the crystal grains at thedepth of 50 min. If this relation is not met, the metal structure of theshell portion would be too non-uniform in a radial direction, meaningthat the wear resistance and the resistance to surface rougheningdecrease rapidly by machining the roll surface to remove a surfaceroughness after a certain period of service. This leads to a high rollcost per a unit amount of rolled steel sheets.

The present invention will be explained in detail by way of thefollowing Examples.

EXAMPLE 1 Comparative Example 1

Using a hollow cylindrical mold having an inner diameter of 420 mm and aroll body length of 1530 mm, 700 kg of a melt having a composition shownin Table 1 was centrifugally cast to provide a sleeve having a thicknessof 60 mm.

                                      TABLE 1                                     __________________________________________________________________________    Sample Chemical Composition (weight %)                                                                          Ts.sup.(2)                                                                       V.sub.av.sup.(3)                         No..sup.(1)                                                                       C  Si Mn P  S  Ni Cr Mo V  W  (°C.)                                                                     (mm/min.)                                __________________________________________________________________________    1   1.41                                                                             0.80                                                                             0.45                                                                             0.021                                                                            0.013                                                                            0.69                                                                             6.01                                                                             1.95                                                                             2.48                                                                             3.00                                                                             1440                                                                              12                                      2   1.38                                                                             0.76                                                                             0.42                                                                             0.023                                                                            0.011                                                                            0.70                                                                             5.97                                                                             1.94                                                                             2.55                                                                             2.98                                                                             1475                                                                             100                                      __________________________________________________________________________     Note:                                                                         *.sup.(1) Sample No. 1: Example 1                                             Sample No. 2: Comparative Example 1                                           .sup.(2) Supply temperature of the melt.                                      .sup.(3) Average shell portionforming speed.                             

In both cases, the inner surface of the mold was coated with arefractory material in a thickness of 2.5 mm, and the rotation speed ofthe mold was set such that a centrifugal gravity number was 140 G on thesurface of the melt being formed into the shell portion in the mold. Byconducting a differential thermal analysis, the primary crystal-formingtemperature Tc was found to be 1390° C. Accordingly, the supplytemperature of the melt was Tc+50° C. in Example 1 and Tc+85° C. inComparative Example 1. Thus, the casting of the shell portion wascompleted for 5 minutes. The average shell portion-forming speed wasabout 12 mm/min.

In the method of Example 1, to measure the advance speed of thesolidification interface of the melt, 200 g of iron sulfide was added tothe melt in the inlet opening of the mold when the thickness of the meltsupplied became 10 mm and 40 mm, respectively. The resulting shellportion was cut to obtain test pieces for measuring the metal structureof the shell portion.

FIG. 1 schematically shows the result of the sulfur print test inExample 1. The results of the sulfur print test are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    At Time of Adding                                                                            Position of Solidification                                                                Thickness of                                                                         Average Advance                             Iron Sulfide   Interface Measured                                                                        Melt Pool                                                                            Speed of Solidification                     Time (t)                                                                             T* of Melt (A)                                                                        by Sulfur Print (B)                                                                       (A-B)  Interface (B/t)                             __________________________________________________________________________    50 sec.                                                                              10 mm    8 mm       2 mm   9.6 mm/min.                                 3 min. 20 sec.                                                                       40 mm   33 mm       7 mm   9.9 mm/min.                                 __________________________________________________________________________     Note                                                                          *Thickness.                                                              

As shown in FIG. 1, since the position of the solidification interfaceof the melt was 8 mm from the inner surface of the mold (outer surfaceof the shell portion) when the thickness of the melt added was 10 mm,the thickness of the unsolidified melt pool was 2 mm, and the averageshell portion-forming speed (average advance speed of the solidificationinterface) was 9.6 mm/min (0.16 mm/sec.). Also, since the position ofthe solidification interface of the melt was 33 mm from the innersurface of the mold when the thickness of the melt added was 40 mm, thethickness of the unsolidified melt pool was 7 mm, and the averageadvance speed of the solidification interface was 9.9 mm/min. (0.17mm/sec.).

It was then confirmed by the observation of the metal structure of theshell portion that the shell portion had a fine and uniform matrixstructure from a surface area to a depth of 50 mm.

EXAMPLE 2 Comparative Example 2

Using a hollow cylindrical mold having an inner diameter of 1130 mm anda roll body length of 1593 mm, a melt having a composition shown inTable 3 was centrifugally cast in the same manner as in Example 1 toprovide a sleeve having a thickness of 100 mm.

                                      TABLE 3                                     __________________________________________________________________________    Sample Chemical Composition (weight %)                                                                          Ts.sup.(2)                                                                       V.sub.av.sup.(3)                         No..sup.(1)                                                                       C  Si Mn P  S  Ni Cr Mo V  W  (°C.)                                                                     (mm/min.)                                __________________________________________________________________________    3   1.85                                                                             0.80                                                                             0.70                                                                             0.024                                                                            0.009                                                                            1.08                                                                             4.14                                                                             5.71                                                                             4.27                                                                             2.93                                                                             1385                                                                              15                                      4   1.84                                                                             0.85                                                                             0.67                                                                             0.026                                                                            0.011                                                                            0.99                                                                             4.25                                                                             5.97                                                                             4.31                                                                             3.00                                                                             1420                                                                             100                                      __________________________________________________________________________     Note:                                                                         *.sup.(1) Sample No. 3: Example 2                                             Sample No. 4: Comparative Example 2                                           .sup.(2) Supply temperature of the melt.                                      .sup.(3) Average shell portionforming speed.                             

In both cases, the inner surface of the mold was coated with arefractory material in a thickness of 2.0 mm, and the rotation speed ofthe mold was set such that a centrifugal gravity number was 120 G on thesurface of the melt being formed into the shell portion in the mold. Theprimary crystal-forming temperature Tc was 1335° C. Accordingly, thesupply temperature of the melt was Tc+50° C. in Sample No. 3 (Example2), Tc+85° C. in Sample No. 4 (Comparative Example 2).

Incidentally, the initial supply speed of the melt (corresponding toabout 30% of the total amount the melt supplied) was made high, and thesupply speed of the remaining melt was made low. Namely, in the shellportion having a total thickness of 100 mm, the initially cast (outer)portion of the shell portion having a thickness of 30 mm was formed at asupply speed of 120 mm/min., and the later cast (inner) portion of theshell portion having a thickness of 70 mm was formed at a supply speedof 11 mm/min. in Example 2. Thus, the casting of the shell portion wascompleted for 6 minutes 37 seconds, and the average shellportion-forming speed was about 15 mm/min. in Example 2.

In the same manner as in Example 1, each of the resulting shell portionswas machined to obtain test pieces for measuring the metal structurethereof. Also, after eliminating the surface machining allowance of theas-cast products by machining, the entire roll body (length: 1530 mm) ofeach compound roll was machined at a depth of 25 mm and 50 mm,respectively from a surface to investigate the cast defects andsegregation by an ultrasonic testing method, by observation by the nakedeye, and by macro-etching.

FIGS. 2(a) and 2(b) are photomicrographs showing the metal structure ofthe shell portion in a surface portion and in a deep portion (50 mm fromthe surface) in Example 2, and FIGS. 3(a) and 3(b) are photomicrographsshowing the metal structure of the shell portion in a surface portionand in a deep portion (50 mm from the surface) in Comparative Example 2.In these photomicrographs, areas in a black color are a matrix structure(primary crystal structure), and those in a white color are carbideparticles.

The metal structure of the shell portion in a surface portion and a deepportion (50 mm from the surface) in each compound roll of Example 2 andComparative Example 2 was quantitatively measured by an image analysismethod. In the measurement, crystal grains having various shapes werefirst converted to true circles having the same areas in thephotomicrograph as those of the crystal grains, and only the truecircles having diameters exceeding 30 μm were counted for obtaining theaverage diameter of the crystal grains. For the measurement of thematrix structure, the surface of the test piece to be measured wassubjected to heavy etching so that the matrix particles (primarycrystals) were turned to black, and the same measurement was repeated on20 fields in the photomicrograph. An average value of the measuredresults was used as an average diameter of the matrix particles.

As a result, in Comparative Example 2, the matrix structure (crystalgrains) had an average diameter of 83 μm in a surface portion and 113 μmin a portion as deep as 50 mm from the surface. On the other hand, inExample 2, the matrix structure (crystal grains) had an average diameterof 75 μm and 88 μm, respectively, in a surface portion and in a portionas deep as 50 mm from the surface.

It is clear from the above results, in the shell portion of the compoundroll according to the present invention, the shell portion is formed bya fine and uniform metal structure from a surface to a deep portion.

As a result of ultrasonic testing, no cast defects were observed inExamples 2 and Comparative Example 2. After eliminating the surfacemachining allowance of the as-cast products, the shell portion wasmachined repeatedly by 5 mm in a radial direction to observe the metalstructure of the inside portion of the shell portion. The segregation ofcarbide (1 mm in diameter) was observed by the naked eye not only at onepoint at the depth of 25 mm but also at one point at the depth of 50 mmin the shell portion produced in Comparative Example 2.

As a result of observation by the naked eye and macro-etching, it wasconfirmed that there was a non-uniformity of the metal structure(segregation) in an entire area at a depth exceeding 25 mm in the shellportion of Comparative Example 2. On the other hand, in Example 2, castdefects and segregation were not observed in the shell portion by thenaked eye and by macro-etching.

As described above, the compound roll of the present invention comprisesa shell portion having a fine and uniform metal structure free from castdefects, segregation, etc. Therefore, it can be used for producinghigh-quality rolled steel sheets by hot rolling or cold rolling, and theamounts of the rolled steel sheets per a unit consumption of the shellportion of the compound roll can be increased.

What is claimed is:
 1. A method of producing a compound roll comprisinga shell portion made of a hard, high-alloy cast steel or iron havingexcellent wear resistance and resistance to surface roughening and acore portion made of a tough cast iron or steel, said high-alloy caststeel or iron of said shell portion having a composition consistingessentially, by weight, of 1.0-3.0% of C, 2.0% or less of Si, 2.0% orless of Mn, 2.0-15.0% of Cr, 10.0% or less of Mo, 2.0-8.0% of V, thebalance being substantially Fe and inevitable impurities, an averagediameter of crystal grains constituting a matrix of the metal structureof said shell portion being 100 μm or less in a range from a surface toa depth of 50 mm when determined by an image analysis method on thecrystal grains having diameters exceeding 30 μm, and said crystal grainssatisfying the formula: m₂ ≦1.2 m₁, wherein m₁ is an average diameter ofsaid crystal grains at the surface of said shell portion, and m₂ is anaverage diameter of said crystal grains at the depth of 50 mm, saidmethod comprising centrifugally casting said shell portion by supplyinga melt for said shell portion at a temperature T satisfying the formula:Tc≦T≦Tc+90° C., wherein Tc is a primary crystal-forming temperature ofthe melt for said shell portion, to a hollow cylindrical mold rotatablearound its longitudinal axis, at such a speed that an average shellportion-forming speed in said mold is 2-40 mm/min.
 2. The methodaccording to claim 1, wherein said melt for said shell portion issupplied to said hollow cylindrical mold in two stages comprising afirst stage wherein said melt is supplied to said hollow cylindricalmold at such a speed that said average shell portion-forming speed is50-200 mm/min. until up to about 40% of the total thickness of saidshell portion is formed, and a second stage wherein said melt issupplied to said hollow cylindrical mold at such a speed that saidaverage shell portion-forming speed is 2-40 mm/min. until the formationof the remaining portion of said shell portion is completed.
 3. Themethod according to claim 1, wherein said high-alloy cast steel or ironof said shell portion further contains 20.0 weight % or less of W. 4.The method according to claim 1, wherein said high-alloy cast steel oriron of said shell portion further contains 3.0 weight % or less of Ni.5. The method according to claim 1, wherein said high-alloy cast steelor iron of said shell portion further contains 10.0 weight % or less ofCo.
 6. The method according to claim 1, wherein said high-alloy caststeel or iron of said shell portion further contains 0.03-0.2 weight %of N.
 7. The method according to claim 2, wherein said high-alloy caststeel or iron of said shell portion further contains 20.0 weight % orless of W.
 8. The method according to claim 2, wherein said high-alloycast steel or iron of said shell portion further contains 3.0 weight %or less of Ni.
 9. The method according to claim 2, wherein saidhigh-alloy cast steel or iron of said shell portion further contains10.0 weight % or less of Co.
 10. The method according to claim 2,wherein said high-alloy cast steel or iron of said shell portion furthercontains 0.03-0.2 weight % of N.